I find it weird people don’t have mustard in their pantry, I have English, Dijon, whole grain and horseradish. If the first couple liked the mustard but didn’t have any in their pantry I’d leave it there.
I dunn 'bout the social niceities, but…
after 20 posts, why hasn’t anybody yet mentioned the thread title/usename combo? 
The tough part is picking out a** mean **mustard. It’s more challenging than picking out a wine.
Add me to the “it’s clueless” list. Presumably you have leftover corned beef and can continue enjoying what should have been a contribution to the potluck and stayed at your house.
Then it’s weird.
But still not a big deal. And sort of practical. I have a lot of old salad dressing and stuff that people brought to pot luck parties and left behind. They aren’t things we especially like, so they hang around until they aren’t food any more, then get tossed.
Psst…I did, in post 14, and it was awesome.
The hosting couple can send it back, but the guest couple should not, ideally, simply take it back.
With that said it’s a minor point and the main theme of politeness is making people feel comfortable so it’d be silly to get worked up over something like this. (Not that anyone has.)
Ehh probably a childhood trauma of grandma yelling at him for forgetting to bring back the cake pan left an instinct to always bring back the stuff you took.
Except the toilet paper.
Both. Neither. People have all sorts of funny attitudes about lots of things and food seems to be a common one. See I would have left it. I once went somewhere for dinner and took 3 dipping sauces I knew they would never have seen and left them all. But I wouldn’t be surprised that someone took home something that they know I don’t keep in my pantry, why waste it?
ETA And wolfman’s hypothesis may be right even if C2 doesn’t recall it.
Psst…To both of you. It was mentioned in Post One 
mmm
Pffft–who reads the OP?
Did he only bring the mustard? No wine, no dessert, etc.?
It’s a frigging bottle of Dijon mustard, ordinariness personified (if a bottle of mustard had personhood).
Now if he’d brought and then appropriated a bottle of Moutarde de Meaux, that’d be rude.
Yes, mustard only.
If it were me I would have felt foolish retaining possession of the mustard after having brought it to the dinner. I probably would have said something like, “I’ll leave this with you since you both enjoyed it; you can use it with the leftovers”.
mmm
What you didn’t mention in your OP, at least not explicitly, was that you yourself were a participant in the dinner party of which you spoke, your story being told from the Point of View of the mustard.
That’s what Left Hand of Dorkness was referring to in Post #14.
That was intentional.
mmm
It seems most people (in the world, not just this thread) think that their personal rules of etiquette are ‘‘the norm’’. People genearally assume too quickly and don’t communicate well enough. Hence the scandalous episode with the mustard.
I throw a large New Year’s Eve party most years. I serve a full supper and snacks, but many of the guests also bring food and drink. What happens at the end is mixed. Some people leave their stuff, some take it home, and some ask me if I want the leftovers as they gather up their dishes. Probably more leave the food than not, but there isn’t a single uniform norm, in my experience.
Oh–I thought this line was missing a “t”:
Now my joke looks lame :(.
My take is kind of a mix. If these folks have been friends for a long time they may have a very practical relationship. Not one based on standard etiquette as much as their common experience.
e.g. If you visit my house you need to bring your own mustard. I don’t have any; never have, and never will. You’re welcome to bring you own and if you try to leave it I’ll invite you to take it with you. Not out of some unwillingness to accept a gift but simply that neither of us is better off if you pay for something I’m going to throw away uneaten. How about we make both of us happier: you get to eat it later and I don’t have to feel guilty pitching food you paid for?
That’s how relationships between actual *friends *work. Interactions with *acquaintances *are more stereotyped, with conventional courtesy & conventional expectations followed. So the guest leaves the mustard unremarked, and the host(ess) accepts it unremarked. What will / did happen to the mustard later is discreetly kept a secret.
From the OP we don’t know the relationship between C1 & C2. Nor which role mmm played.